Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
Description of the Related Art
In recent years, what is called a belt-pad type image forming apparatus has been employed in the related art. In the belt-pad type image forming apparatus, a nip portion is formed by involving a fixing belt and bringing a pad (pressing member) disposed inside the fixing belt into contact with a pressurizing roller configured to pressurize.
In this fixing device, the fixing belt is driven to rotate with the pressurizing roller so as to fix unfixed toner on a sheet and convey the sheet to a sheet ejection tray. At that time, in order to smooth the rotation of the fixing belt, a lubricant is interposed between the inner periphery of the fixing belt and the pad so as to reduce sliding resistance.
However, compared to what is called a belt-roller type fixing device in which a roller instead of a pad is brought into contact with the inner periphery of a fixing belt, the belt-pad type fixing device has a disadvantageous short lifetime since the inner periphery of the fixing belt and the pad are slid, which exhausts the lubricant when being used for a long period.
Therefore, in the belt-pad type fixing device, it is necessary to prevent exhaustion of the lubricant in order to secure a long lifetime.
The following documents are examples of one disclosing a fixing device configured to supply a lubricant between a pad and the inner periphery of a fixing belt in order to prevent exhaustion of the lubricant: that is, JP 2014-174434 A, and JP 2013-68888 A.
In the fixing device disclosed in JP 2014-174434 A, a sheet-like sliding sheet containing a lubricant is mounted on a surface of a pressing member disposed in a fixing belt. The sliding sheet includes a fiber sheet and the like. This sliding sheet is provided with inclined creases for flowing the lubricant in one direction. Furthermore, a pipe that forms a circulation channel for circulating the lubricant is connected to one end and the other end of the sliding sheet. A container configured to accumulate the lubricant and provided outside the fixing device is connected to the pipe. The lubricant is pressed into the pipe from the container with a pump and is circulated within the circulation channel. The lubricant that has reached one end of the sliding sheet moves toward the other end of the sliding sheet along the creases and is introduced into the pipe connected to the other end.
In the fixing device disclosed in JP 2013-68888 A, a sliding sheet is interposed between a pressing member disposed in a fixing belt and the inner periphery of the fixing belt. The sliding sheet is conveyed in a reciprocating manner by a conveyance system. A moving direction of the sliding sheet differs between a driving mode and a cleaning mode. In the cleaning mode, a surface of the sliding sheet in a side close to the fixing belt is cleaned by a cleaning roller, and the cleaned surface is supplied with a lubricant by a lubricant supply roller. After the lubricant is supplied to the sliding sheet, the cleaning roller is separated from the sliding sheet, and the sliding sheet maintains in a standby condition until changing to the driving mode. In the driving mode, the sliding sheet moves toward a position between the pressing member and the fixing belt so that the lubricant is supplied between the fixing belt and the pressing member.
However, the fixing device disclosed in JP 2014-174434 A is configured to pour a lubricant with a pump, which may bring about pulsations in a circulation channel and uneven supply of the lubricant. Furthermore, such a fixing device requires a lubricant to fill inside a pipe so that an amount of the lubricant held in a container is increased. Still further, in a case of supplying semi-solid grease having high viscosity as a lubricant, a high pressure is required for moving the lubricant inside the thin pipe so that the pump grows in size. Therefore, the abovementioned fixing device is unsuitable when using a highly viscous lubricant.
In the fixing device disclosed in JP 2013-68888 A, a lubricant should be supplied to a sliding sheet over a range where the sliding sheet is to be conveyed, which increases an amount of the lubricant to be held by a lubricant supply roller. Furthermore, it is difficult for the sliding sheet to evenly hold the lubricant supplied over a wide range, which causes unevenness in an amount of the lubricant supplied between a pressing member and a fixing belt.